In such a scanner scanning may be obtained by moving the scanner instead of using an automatic scanning geometry. Under application of simple optical elements, such as an optical fiber or light collector, the light, in general from an incoherent light source illuminates the object and light scattering therefrom is collected through, for instance, an optical fiber by a sensor. Generally, scanners of this type can operate without imaging optics and during the scanning the scanner is contacting the object, e.g. a symbol code surface.
Now in order to obtain non-contact reading some scanners use simple optical imaging means. However, due to the relative compact size and its use often of a low-cost incoherent light source, the associated depth of field is very limited. Moreover, in practical use, the scanner is moved opposite the object surface on a distance of several millimeters to several centimeters due to the fact for example that the object, e.g. a symbol code, is behind a glass or foil cover. The range then in which the object has to be investigated or read should be as large as possible. This means that the depth of field for the illuminating optics or the imaging optics should be as large as possible. Even in the case that a coherent light source is used, like a laser for which the depth of field of the optics can be less limited, there is always the requirement of having a working or reading range as large as possible.
In general, scanners can be classified in two types of operation. In the one type, the so called active mode type, a thin beam pencil is used to scan the object. The light scattering therefrom is detected in order to derive information therefrom. In order to form this beam pencil a focussing optics has to be used. In the other mode type the so called passive mode is used. In this type, the object is imaged onto a sensor by means of an imaging optics and the information of the object surface is detected by this sensor in ambient light with or without additional illumination.
Upon scanning the object like a symbol code with a light beam the size of the scanning point on the bar code should be small enough to distinguish the narrow symbol or bar. In a passive scanner, the resolution of the imaging optics has to be sufficient to distinguish the smallest object information or bar width, the size of the spot on the photo sensor should be smaller than or equal to the bar width. The higher is the resolution of the illuminating optics or imaging optics, the thinner is the bar code that can be read. The requirements for high spatial resolution and large depth of field cannot, however, be met simultaneously. The high spatial resolution of an optical system leads to small depth of field. Said depth of field is proportional to the relative optical lens aperture and inversely proportional to the size of the image spot. Thereby the depth of field is limited by the required spatial resolution.